When a mirror is situated in a hot, high humidity environment such as a bathroom or kitchen, it is particularly susceptible to the formation of a mist or condensed water on the exposed surface. This is due to the condensing action of the cold mirror surface on the steam or water vapor in the vicinity of the mirror resulting in condensation on the front surface of the mirror.
There have been examples of heated or demisting mirrors in the prior art, but none of the prior art approaches has been commercially successful because of their performance shortcomings. If a demisting mirror is to solve the problem facing the user in the bathroom in normal operating conditions, it must heat up quickly from a relatively cold condition, and it should not use an excessive amount of power. In addition, there are severe safety requirements because of the hazards of using electrical appliances in areas where the user is usually wet and well grounded, raising the risks of electrocution from any leakage currents associated with such appliances.
Examples of prior art anti-condensation mirrors are shown in the patents to Seibel, et al. U.S. Pat. No. 3,887,788; Seibel, et al. U.S. Pat. No. 3,839,620; Rust U.S. Pat. No. 3,530,275; and Catterson U.S. Pat. No. 3,160,736. The Seibel, et al. patents both disclose heating elements on printed circuit boards with various types of associated control means. The wattage limitations in the printed circuit board approach would make this impractical insofar as providing a heater which could bring the mirror up to the desired temperature in a relatively short time. Any printed circuit boards capable to providing relatively high wattage heat to the mirror would be relatively expensive, and the proper controls for any such arrangement would be difficult. In one embodiment of the Seibel, et al. U.S. Pat. No. 3,887,788 no temperature control is provided but it is acknowledged that it would be necessary to restrict the wattage and have a slow heat-up time with such an approach.
The patents to Rust and Catterson cited above are characterized by relatively complex arrangements of laminated glass and layers of insulation which support exposed resistance wire elements. It would be difficult to provide sufficient wattage for quick heat-up to elements of this design. The large thermal mass involved requires high wattage to attain the desired temperature for maintaining a mist free mirror and the exposed wire elements enclosed behind glass laminates would be hazardous in the event of glass breakage and from the standpoint of moisture possibly entering the area where the exposed resistance wire elements could be shorted or produce leakage currents.
There have been some examples in the prior art of heated mirrors for automobiles that have used positive coefficient resistance materials either as temperature control elements for constant resistance heaters or as heating elements. Noted in this connection are the U. S. patent to Berg, et al. U.S. Pat. No. 4,410,790 and the published European Patent Application No. 0054901.